Major Depressive Disorder (MDD)
Body System: Nervous System | Area Affected: Brain
Doctor to Consult: Psychiatrist, Psychologist, Primary Care Physician
What It Is
Depression, or Major Depressive Disorder (MDD), is a common and serious medical illness that negatively affects how you feel, the way you think, and how you act. It causes persistent feelings of sadness and/or a loss of interest in activities you once enjoyed. It can lead to a variety of emotional and physical problems and can decrease your ability to function at work and at home.
Why It Happens
The exact cause of depression is not fully understood, but it's believed to involve a complex interplay of genetic, biological, environmental, and psychological factors. Imbalances in brain chemicals (neurotransmitters like serotonin, norepinephrine, and dopamine), structural differences in the brain, hormonal changes, chronic stress, and traumatic life events can all contribute to its development.
Who Gets It / Risk Factors
Risk Factors
Family history of depression
Major life changes or trauma (e.g., loss of a loved one, job loss)
Chronic illness or pain
Substance abuse
Certain medications
Personality traits (e.g., low self-esteem, pessimism)
Female gender (higher prevalence)
When to Seek Emergency Care
Thoughts or plans of self-harm or suicide
Inability to care for oneself (e.g., not eating, not drinking)
Experiencing psychosis (hallucinations, delusions)
Severe agitation or aggression
Medical Overview
A professional summary of diagnosis, treatment, and prevention.
Persistent sad, anxious, or 'empty' mood
Loss of interest or pleasure in hobbies and activities
Fatigue and decreased energy
Difficulty concentrating, remembering, or making decisions
Insomnia or hypersomnia (sleeping too much)
Changes in appetite or weight
Feelings of worthlessness or guilt
Thoughts of death or suicide
Restlessness or irritability
Aches or pains, headaches, cramps, or digestive problems without a clear physical cause
Scientific References
This content is based on established clinical guidelines and public health resources from the following authoritative sources.
Medically Reviewed for Accuracy
Medical Advisory Board
•Last reviewed: March 20, 2026Content is aligned with established clinical guidelines from authoritative medical institutions, including MedlinePlus (U.S. National Library of Medicine), the CDC, the NIH, and the NHS. All content is reviewed by our medical advisory board for accuracy and safety.